mash: Migrate ScopedTargetRootWindow to //ash/common

ash/common/wm_shell.h

Index: ash/common/wm_shell.h
diff --git a/ash/common/wm_shell.h b/ash/common/wm_shell.h
index 79968ecf6f5c37f1942d14d18ca44d4879a8d057..176cfc8e8f57c5c11bf35bf118c52c41ad77449e 100644
--- a/ash/common/wm_shell.h
+++ b/ash/common/wm_shell.h
@@ -38,6 +38,7 @@ class MaximizeModeController;
 class MruWindowTracker;
 class NewWindowDelegate;
 class ScopedDisableInternalMouseAndKeyboard;
+class ScopedRootWindowForNewWindows;
 class SessionStateDelegate;
 class ShelfDelegate;
 class ShelfModel;
@@ -150,6 +151,10 @@ class ASH_EXPORT WmShell {
   // appropriate container in the returned window.
   virtual WmWindow* GetRootWindowForNewWindows() = 0;
 
+  // Allows a temporary override of the result of GetRootWindowForNewWindows().
+  virtual std::unique_ptr<ScopedRootWindowForNewWindows>
+  CreateScopedRootWindowForNewWindows(WmWindow* root) = 0;
+
   // Retuns the display info associated with |display_id|.
   // TODO(msw): Remove this when DisplayManager has been moved. crbug.com/622480
   virtual const DisplayInfo& GetDisplayInfo(int64_t display_id) const = 0;